The present invention relates to an improvement in check valve. In a conventional check valve, there is provided a separating wall member in the housing which has an inlet and an outlet ports connected with outer conduits for directing the fluids, such as air, water, oil and the like. The separating wall member has plurality of openings to pass the fluid therethrough and is usually disposed perpendicularly with respect to the flowing direction of the fluid. The separating wall member usually has a valve member which is made of rubber and fixed to the wall member at its root portion to cover the aforementioned openings. The peripheral portion of the conventional valve member keeps a contact with the wall member in its free condition.
In that case, the fluid can freely flow in one direction through the openings, since the valve member is lifted by the pressure of the fluid. However when the fluid flows in the reverse direction, the periphery of the valve member is pushed onto the separating wall member by the fluid pressure thereby to close the aforementioned openings, so that the fluid flow in the reverse direction is completely cut off.
Therefore, the above check valve is not applicable to specific devices for exhaust gas purification, since the devices require various complex controls. In fact, especially in the vacuum control devices for exhaust gas purification, there is required a special check valve which allows free flowing in one direction, cutting off the flow in the reverse direction under a big pressure difference, while allowing a small flow in the same reverse direction under a small pressure difference.